# OpenMP backend (2D FDTD)

The **OpenMP backend** distributes spatial field updates across CPU threads. Its first optimization level shares rows between threads while keeping the magnetic and electric phases ordered:

```cpp
for (std::size_t n = 0; n < advance.nsteps; ++n) {
    #pragma omp for schedule(static)
    for (std::size_t j = 0; j < rows; ++j) {
        stepper.h_phase(j, n);
    }

    #pragma omp for schedule(static)
    for (std::size_t j = 0; j < rows; ++j) {
        stepper.e_phase(j, n);
    }
}
```

This loop runs inside one parallel region. Each `omp for` ends with an implicit barrier, so the electric phase reads completed magnetic values.

`v2` adds SIMD inside row updates. `v3` reuses one coefficient tuple when the interior is uniform. `v4` changes the sweep to thread-owned bands, bringing magnetic and electric work closer together. Compare these levels with the same grid and thread count; the versions change execution strategy while retaining the numerical model.

Source: [fdtd_tmz_backend_omp_v1.cpp](https://github.com/yanevskiv/fdtd-cuda/blob/main/src/fdtd/TMz/backends/omp/fdtd_tmz_backend_omp_v1.cpp).